The present invention relates to a fuel injection system of an internal combustion engine for calculating a fuel injection amount based on a suction air amount of the engine which is detected by an air flow meter. Particularly, the invention relates to a fuel injecting method in which a standard fuel injection amount is corrected based on an altitude compensating learning correction factor corresponding to a change in an atmospheric pressure.
Generally, as the altitude becomes higher, i.e., as the air pressure becomes lower, the measured value of suction air detected by the air flow meter becomes larger than the actually admitted amount. In order to correct the detected value of the suction air amount, a control method has been proposed. It is such that a learning correction factor corresponding to a change in atmospheric pressure is stored in a memory of a backup RAM and a standard fuel injection amount is corrected based on the learning correction factor. The learning correction factor is always updated during engine operation. In this method, however, the learning correction factor stored in the memory is erased when a battery is demounted from the vehicle for replacement or charging so that feeding to the backup RAM is discontinued. When the engine is started after mounting the battery, therefore, it is impossible to immediately execute the fuel injection amount control based on the learning correction factor. Consequently, much time is required until the optimum fuel injecting amount corresponding to the atmospheric pressure is determined again.
To cope with this problem, a learning factor controlling method for an internal combustion engine has been disclosed in Japan published unexamined patent application No. 61-28739. In this art, a computer detects discontinuation of feeding to the backup RAM due to disconnection from the battery. When the battery is connected and the engine s started again, the computer acts to expedite the updating time interval of the learning factor so as to immediately restore the appropriate value of the learning correction factor. This art, however, includes another problem. Since the optimum amount of fuel corresponding to the atmospheric pressure cannot be injected from the initial starting of the engine, the air/fuel ratio cannot be controlled to immediately approach a desired air/fuel ratio, e.g., the stoichiometric air/fuel ratio.